Vertical-cavity surface-emitting devices have generally used an oxide aperture to funnel current into small active volumes of their optical cavities. The oxide aperture has been successfully used to create an aperture close to the device's active region and produce high efficiency through both mode and current confinement. However oxide vertical-cavity surface-emitting lasers (VCSELs) are limited by internal strain, heat flow, and manufacturing non-uniformity in the oxide aperture size that results from the oxide formation.
RCLED's are described in U.S. Pat. No. 5,226,053. A RCLED is a light emitting diode (LED) that generates mainly spontaneous emission and generally operates without a distinct threshold. Resonant cavity light emitting diodes (RCLEDs) also use oxide apertures and operate in the spontaneous emission regime. Oxide-aperture RCLEDs suffer similar problems to oxide-aperture VCSELs due to high internal strain, self-heating, and manufacturing non-uniformity.
While the oxide aperture has been successfully used in many VCSEL devices, it has drawbacks due to its material differences between the oxide material used to form the aperture, and the surrounding semiconductor material. The oxide generally has a different thermal expansion coefficient than the oxide, and proceeds through a timed diffusion process that results in aperture size variation across a processed VCSEL wafer. The oxide aperture is also limited in where it can be placed in the vertical cavity to avoid or minimize material degradation and strain inherent in the oxide.
Other techniques such as buried tunnel junctions or proton implanted resistive regions have also been used to obtain current and mode confinement without the need for an oxide aperture. However the tunnel junction can lead to increased resistance and voltage drop at the high current density used for the vertical cavity light source. The proton implanted VCSEL suffers from the thick implanted region required to achieve electrical isolation, which also increases the electrical resistance. Poor optical mode behavior also results due to self-heating and minimal built-in optical guide for the lasing mode.
Therefore the technology of vertical cavity surface emitting devices has a remaining need for device that can provide epitaxial mode confinement, while being able to engineer the mode confinement, electrical injection, surface step height, and material quality in producing the device.